1. Field of the Invention
The invention relates to an apparatus and method for diagnosing catalyst deterioration. More particularly, the invention relates to a catalyst deterioration diagnosing apparatus and method which takes into account the effect from sulfur in fuel used in an internal combustion engine.
2. Description of the Related Art
An internal combustion engine for a vehicle, for example, is provided with a catalyst for purifying exhaust gas arranged in its exhaust system. Some catalysts have the ability to store oxygen (i.e., an O2 storage function) which adsorbs and retains excess oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst is greater than the stoichiometric air-fuel ratio, i.e., lean, and releases the adsorbed and retained oxygen when the air-fuel ratio of the exhaust gas flowing into the catalyst is less than the stoichiometric air-fuel ratio, i.e., rich. For example, in a gasoline engine, air-fuel ratio control is performed so that the air-fuel ratio of the exhaust gas flowing into the catalyst approaches the stoichiometric air-fuel ratio. However, when a three-way catalyst with the ability to store oxygen is used, even if the actual air-fuel ratio is off somewhat from the stoichiometric air-fuel ratio due to the driving conditions, the oxygen storing/releasing action of the three-way catalyst is able to absorb the air-fuel ratio difference.
Incidentally, when the catalyst deteriorates, the control (i.e., purification) efficiency of the catalyst decreases. Meanwhile, there is a correlation between the degree of deterioration of the catalyst and the degree to which the oxygen storage function has decreased, due to a reaction via a precious metal. Therefore, deterioration of the catalyst can be detected by detecting a decrease in the oxygen storage function. Typically, a method for diagnosing deterioration of a catalyst (i.e., a so-called Cmax method) is performed in which active air-fuel ratio control is performed which forcibly switches the air-fuel mixture in the combustion chamber, and thus the air-fuel ratio of the exhaust gas that flows into the catalyst, so that it is rich or lean, and then the oxygen storage capacity of the catalyst following execution of this active air-fuel ratio control is measured.
For example, Japanese Patent Application Publication No 6-159048 (JP-A-6-159048) describes an apparatus which i) switches the exhaust gas supplied to a catalyst from lean to rich with respect to the stoichiometric air-fuel ratio, ii) calculates the amount of oxygen released from the catalyst during a period of time from the switch until an output signal from an air-fuel ratio sensor downstream of the catalyst becomes rich, based on the total amount of exhaust gas that flowed through the catalyst during that period of time and the air-fuel ratio of exhaust gas supplied to the catalyst during that period of time, iii) recognizes this amount of oxygen as the ability of the catalyst to store oxygen, and iv) detects the degree of catalyst deterioration based on that ability to store oxygen.
Meanwhile, the fuel may contain a relatively high concentration of sulfur (S) depending on the (geographical) region where the internal combustion engine is being used and the like. When such fuel is used, the sulfur components accumulate in the catalyst, causing poisoning (S poisoning). When S poisoning occurs, the oxygen storage and release reaction of the catalyst becomes impeded, which reduces the apparent oxygen storage capacity of the catalyst. However, the catalyst recovers from the poisoning over time when fuel having a low sulfur concentration is used again. In this way, the decrease in performance of the catalyst due to S poisoning is temporary. Therefore, when diagnosing catalyst deterioration, it is important that this temporary decrease caused by S poisoning not be erroneously diagnosed as a malfunction or permanent deterioration such as heat deterioration that is normally supposed to be diagnosed. It is particularly important not to erroneously diagnose a catalyst that is operating normally but on the border between normal and deteriorated as being deteriorated.